I've come to the conclusion that it's Avis's policy to just charge you extra for something every so often without real rhyme or reason - it must be how they fund their Christmas party - and there seems to be nothing you can do to prevent it aside from not using Avis.

My particular favorite was the letter they sent me issued on 25th December (festive!) stating that "on secondary inspection" they had found £600 of damage to the door of a car I'd returned over 3 months earlier (which was undamaged and had been locally inspected and closed at the time - which I'd come to insist after a string of other utterly factitious post return charges).

I've found I've used Avis a lot less since then - aside from one recent mistake - and then they decided to charge the prepaid fuel top up which I never wanted and despite the fact the tank was filled minutes before the return.

Strangely I've never had any similar experience with any of the other firms....